Cage link to lice in wild game fish

NEW research points to fish farm cages as most likely source of sea lice, says an organisation calling itself the Sea Trout Group.

Two newly-published studies show links between sea lice infestation on salmon farms and ‘pools’ of sea lice juveniles ready to infect wild fish on a Scottish sea loch.

The studies – both published in Aquaculture Research – were carried out by FRS (Fisheries Research Service) scientists.

After studying lice infestation in farm cages on Loch Torridon, scientists David Hay and Margaret McKibben found that high levels of juvenile lice at the mouths of local salmon rivers coincided with lice on farm fish in the second year of the typical two-year salmon production cycle. These high levels of lice also coincided with periods following those when the farm stock were infested with gravid females – i.e. female lice about to lay eggs. During the sampling period, when there were no gravid female lice on the local fish farm, there were none found at the shore.

Perhaps the most far-reaching conclusion of the study is that the existence of gravid female lice on fish farms exerts an influence far outside the immediate area of the cages.

“The distances between the nearest fish farms and shoreline sampling sites suggest that larval sea lice can be dispersed over distances of at least 4.6 km,” found Hay and McKibben.

“In the absence of synchronised sea lice treatments on all fish farms within a loch system, farms closer than 4.6 km could be re-infected by larval sea lice circulating from other farms, reducing any long-term advantage from treatment. This finding emphasises the need for integrated fish farm area management,” concluded Hay and McKibben.

“The Sea Trout Group believe that this strengthens our case in calling for routine, synchronised anti-sea lice treatment of all farmed salmon,” said STG spokesperson Fiona Cameron. “Indeed, since there is virtually no scientific knowledge of the distances over which sea trout range up and down our coasts, we believe that the only sensible approach is to synchronise treatment over as wide an area as possible, and not simply base this on loch systems.

More than a third of a million gravid lice on one farm

“The other newly-published study, by FRS scientists including Michael Penston, also underlines just the scale of the problem we’re talking about. When we hear that a fish farm only has one or two gravid lice per fish, that doesn’t sound very damaging. However, the FRS team estimated that at the beginning of November there were more than 350,000 gravid lice on one fish farm studied. Shortly after this, a ‘pulse’ of larval lice was recorded in the loch. The scientists believe that the most likely origin of these was the fish farm,” said Ms Cameron.

“The other major factor is that these studies show that lice larvae become concentrated in the areas around river mouths – just the places where sea trout and wild salmon smolts emerge. The sea trout, which remain in coastal waters rather than heading out to the ocean, have no chance of avoiding infestation in such circumstances.

“The scientists suggest that their work provides the ‘first tentative evidence’ that lice from a fish farm were the source of a concentration of larvae at the mouth of a sea trout river. However, the STG believes that we cannot afford to ignore such evidence. The Scottish Executive must act – and act promptly – to ensure that fully-synchronised sea lice management strategies are put in place. This must apply not just in lochs covered by Area Management Agreements, but up and down the entire coast, and in the islands – wherever salmon are farmed.

“The scientists conclude that further research is required. We would not dispute that this is true, but we maintain that, if our irreplaceable stocks of wild salmon and sea trout are to be saved for future generations, we cannot afford to spend further time gathering information before action is taken. Each season which sees second-year salmon carrying sea lice in cages anywhere within 5 km of salmon and sea trout rivers presents a very real and substantial threat to our wild fish – so action is needed now,” added Ms Cameron.

“The Scottish Executive is moving towards introducing new legislation governing aquaculture, and we believe that it is absolutely necessary that this includes provision for adequate regulation, monitoring and policing of sea lice management. It is no longer acceptable, in the current climate of paying lip-service to ‘sustainability’ and the protection of the marine environment, to have a situation where there is no compulsion for fish farmers to treat sea lice infestations adequately or timeously, and no sanctions against any who fail to do so. We also need to establish a properly co-ordinated planning system for all marine activities, including aquaculture, so that tourism, fish farming, shellfish farming, fishing, and other activities take place in harmony rather than conflict.

“We are now seeing the results of three decades of development of fish farming in what has been a basically uncontrolled way. Studies such as the newly-published ones demonstrate that the effects of sea cages are likely to be spread over much larger areas than has been thought. Despite undertakings on the part of the Executive that a list of fish farms which were to be relocated, due to being on unsuitable sites, would be published last year, we are still waiting for a single fish farm relocation to be announced,” concluded Ms Cameron.

The Sea Trout Group (STG) is a group of angling enthusiasts supported by the Salmon & Trout Association (S&TA) and Scottish Anglers National Association (SANA). The group is campaigning for the protection and restoration of stocks of salmon and sea trout in Scotland, particularly in regard to minimising damage caused by marine aquaculture.